Schools in Bristol will be forced to increase class sizes, cut subsidy to school trips and get rid of teachers as the Government cuts some £34million to its funding.

Bristol is likely to lose an average of £677 per pupil under the Government's new National Funding Formula, which it says will help resolve "unfair" funding in previous years.

But it means Bristol's schools will lose nearly £34million in vital funding, according to the six biggest education unions in the country.

It would also be the hardest hit local authority area in the country by 2020, and could mean the loss of more than 1,100 teachers.

The average loss nationally per pupil is £339 for primary and £477 for secondary school children.

Jamie Barry, headteacher of Parson Street Primary in South Bristol, is already facing the huge real-time cuts to his school budget.

He has since written to parents to explain the difficulties the school will face in the coming years.

Under the new formula, he is facing a reduction of £612 per pupil, and by 2019 the loss of seven teachers at his school.

He said: "I'm funding some children at a deficit of £10,000 – which means some pupils are costing £10,000 more than the funding they get.

"We were already facing significant costs before the new formula, and how to deal with it all at the school, and where savings are going to be made.

"Staffing is about 80 to 85 per cent of a school's budget, and that is the key. We no longer subsidise school trips and we've had to cut music provision at the school as well.

"I might be forced to cut staff, which I really don't want to. A lot of schools are facing this problem, and there are some thinking about having four-day weeks.

"School leaders work in exceptionally challenging circumstances every single day. They not only have to provide a curriculum and ensure outcomes, they also have to be concerned with the whole child.

"Staff in our schools are doctors, police officers, social workers, family support assistants and teachers.

"The role is complex and demanding and requires investment which is clearly not there at the moment."

The Department for Education insists school funding is at record levels, with £40billion a year and another £2.5billion for disadvantaged children.

But they say the current system for distributing money is "outdated, inefficient and unfair".

Kevin Courtney, General Secretary of the National Union of Teachers, said: "Schools are already on their knees trying to make ends meet. Budgets have been cut to the bone and decisions such as increasing class sizes and losing staff have already been made.

"To avert this national scandal, Government must reassess its plans and make substantial new funding an urgent priority so that all schools have sufficient money to run an effective education system."

Russell Hobby, General Secretary of the National Association of Head Teachers, added: "School budgets are being pushed beyond breaking point.

"The government's £3 billion real terms cut to education funding must be reversed or we will see education and care suffer. Already heads are being forced to cut staff, cut the curriculum and cut specialist support.

"A new funding formula is the right thing to do, but it cannot be truly fair unless there is enough money to go round in the first place."